A typical erasable semiconductor memory cell is erasable by application of ultraviolet (UV) light thereto. In furtherance thereof, the cell is covered by a topside structure which is transparent to UV light. Such a topside structure of a prior art device is shown in FIG. 1.
A silicon substrate 10 is provided which incorporates typical memory cell structure (not shown) which may be erased by application of UV light thereto. The substrate has provided thereover field oxide (SiO.sub.2) 12 and a layer of BPSG or BPTEOS glass 14. An aluminum layer 16 is patterned over the resulting structure. A relatively thin silicon oxynitride [SiO.sub.x N.sub.y (H.sub.z)] layer 18 is provided over the metal layer 16 in conformance therewith, so that the oxynitride layer 18 is in contact with the metal layer 16, and the glass layer 14, and also, at the edge of the die, the substrate 10. In order to planarize the device, spin-on-glass (SOG) 20 is provided over the resulting structure, substantially filling in the recesses of the structure. The thin oxynitride layer 18 is included to keep the SOG 20 from attacking and reacting with the aluminum, thereby acting as a barrier therebetween. During the planarization step, a very thin layer of SOG 20A, approximately 100 .ANG. thick, is applied to and remains over portions of the thin oxynitride layer 18.
Then, a relatively thick oxynitride layer 22, approximately 12000 .ANG. thick, is provided over the resulting structure, and plastic packaging material 24 is provided over the oxynitride layer.
Although this structure achieves relatively smooth topology, a crack may occur in the SOG layer for example at 26 near the edge of the die and may spread through and along the thin SOG layer 20A. Since the thermal expansion coefficient of the plastic material 24 is much greater than that of the material below it, the plastic material 24 expands and contracts much more than the lower structure for a given change in temperature. The adhesion between the thin SOG 20A and the oxynitride layer 22 is relatively weak, while the adhesion between that oxynitride layer 22 and the plastic packaging material 24 is relatively strong. Thus, under change in temperature, cracking can readily occur as described above in and along the thin SOG 20A. Such a crack can extend through the thick oxynitride layer 22 as at 28 because of its relatively poor physical strength. Thus, a device as thus far described may be readily prone to failure.